A haiku poem consists of three lines, with the first and last line having 5 moras, and the middle line having 7. A mora is a sound unit, much like a syllable, but is not identical to it. Since the moras do not translate well into English, it has been adapted and syllables are used as moras.
Haiku started out as a popular activity during the 9th to 12th centuries in Japan called “tanka.” It was a progressive poem, where one person would write the first three lines with a 5-7-5 structure, and the next person would add to it a section with a 7-7 structure. The chain would continue in this fashion. So if you wanted some old examples of haiku poems, you could read the first verse of a “tanka” from the 9th century.
The first verse was called a “hokku” and set the mood for the rest of the verses. Sometimes there were hundreds of verses and authors of the “hokku” were often admired for their skill. In the 19th century, the “hokku” took on a life of its own and began to be written and read as an individual poem. The word “haiku” is derived from “hokku.”
The three masters of “hokku” from the 17th century were Basho, Issa, and Buson. Their work is still the model of haiku writing today. They were poets who wandered the countryside, experiencing life and observing nature, and spent years perfecting their craft.